I personally prefer SLIME over any IEX client I've ever seen, but that's not a limitation of Elixir. Since Erlang has a Smalltalk-esq image just like CL does, the Elixir REPL is in principle as powerful as a Lisp's. In practice CL has lots of additional little things which are nice, like the ability to update instances in place after changing a class or DEFVAR forms which don't clobber their contents on reloads, but for me most of the advantage comes from just how well designed SLIME is.
